numRows() returns int

numRows() returns an int [1] and should thus be compared to an int and
not a string.

This is important for Sync to find the user account on Firefox Mobile
for Android, see owncloud/apps#826

Fixes owncloud/apps#1261

[1] http://pear.php.net/package/MDB2/docs/latest/MDB2/MDB2_Result_Common.html#methodnumRows